Uncertainties in Solar MeasurementsDespite all that scientists have learned about solar irradiance over
the past few decades, they are still a long way from forecasting
changes in the solar cycles or incorporating these changes into climate
models. One of their biggest obstacles has been technology. Because
even the smallest shifts in solar energy can affect climate
drastically, measurements of solar radiation have to be extremely
precise. Instruments in use today still are subject to a great deal of
uncertainty.

The various sensors agree closely in the timing and amplitude of rapid daily variations due to
the passage of individual sunspot groups. The sensors also agree in the amplitude of the 11-year cycle,
but disagree significantly in the decadal average level of the TSIup to 6 watts per
square meter. This difference is larger than the total variation in solar irradiance
in the past 500 years, so a more accurate assessment is needed to study
the Suns impact on climate change. An upcoming NASA research
satellite, the Solar Radiation and Climate Experiment (SORCE), will
carry instruments designed to do just that. (Graph adapted from C.
Frölich of the World Radiation Center in Davos Switzerland)

The total change in TSI over the 11-year
cycle is believed to be 0.1 percent of the Suns total energy on a
yearly average. Individual sunspot events are very accurately reproduced in independent TSI measurements,
so that the relative accuracy on weekly and 11-year time scales is sufficient to characterize
such changes. However, the most accurate estimates of the long-term average TSI are
uncertain by several times the amplitude of the 11 year cycle. This large uncertainty in
absolute calibration of the instruments means that any possible trend from one 11 year cycle
to the next, the most important change for global warming, is not known accurately enough to
even decide whether the trend is positive, negative, or zero. With such data, scientists have
a good approximation of the 11 year cycle, but no real insight into more subtle changes that may
occur over many decades and centuries.

Even larger uncertainties exist for measurements of the amount of
solar radiation that is absorbed by the Earths atmosphere, ocean, and
land. As of now, researchers know that the atmosphere absorbs between
20 and 25 percent of the TSI and that the land absorbs 45 to 50
percent. With solar radiation, a 5 percent difference is huge. A
difference of even 1 percent would completely throw off climate models
of global warming and scientists understanding of convection (warm,
upward moving air currents) in the atmosphere.

The other big problem scientists face is too little data. Even in
instances when solar energy measurements are accurate, researchers
often dont have enough information with which to draw conclusions.
Building models to forecast long term trends, in particular, requires a
tremendous amount of past data on those trends. At this time,
scientists only have roughly twenty years of satellite data on the Sun
an equivalent of just two 11-year cycles. Most of the data
researchers do have on the Sun are for TSI. Relatively very little
data have been gathered on the spectral changes in the Sun. Scientists
havent determined with precision how the fluctuations in the Suns
output of visible wavelengths differ from near infrared or from
ultraviolet. The dearth of spectral data presents another serious
obstacle for climate modelers since distinct wavelengths are absorbed
by different components of the Earths climate system, which react
differently with one another as their energy levels change.